(1) Field of the Invention
The present invention relates to nuclear reactors and particularly to the control of pressurized water type reactors. More specifically, this invention is directed to a reactor control system employing a large number of low worth absorber elements and especially to the means for exercising individual control over the position of each element. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
The conventional prior art approach to reactor control encompasses the use of comparatively few control rods, also known as absorber elements, of high worth. The use of a small number of elements of high worth, in turn, dictates that the control system be capable of moving the absorber elements in steps. Prior art reactor control systems are, accordingly, generally characterized by complexity and lack of flexibility.
In addition to control system complexity, the prior art technique of using high worth absorber elements has weighed against the association of an absorber element with each fuel bundle. This fact, coupled with the step-wise movement of prior art absorbers, has had a tendency to produce undesirable axial distortions in the reactor core; such axial distortions principally being produced as a consequence of partial insertion of absorber elements.
A further disadvantage of prior art control systems has resided in the practice of extending the actuators for the absorber elements out of the pressure vessel, typically through the removable vessel head, in the interest of facilitating access to the actuators for control purposes. The penetration of the reactor pressure vessel by a plurality of actuators has increased both sealing requirements and, when penetration of the vessel head was the mode of access, the difficulty of reactor head removal as is necessary for inspection and maintenance operations such as refueling. Pressure vessel penetration by the actuator means also imposed physical limitations on the number and placement of the absorber elements and additionally prevented the association of an absorber element with each fuel bundle.
In the interest of enhancing system safety, means for preventing absorber element "blow-out" have been required in all reactor installations. While an unlikely occurance, absorber element "blow-out" could occur if an unexpectedly high pressure differential was established across the actuator means employed to controllably withdraw the control rod from the reactor core. In the prior art such "blow-out" protection means have typically included active devices such as hydraulic fuses or mechanical latches which would be installed within the pressure vessel. Periodic testing of such safety devices is generally impossible. Use of an active type safety device which cannot be periodically cycled to insure operability is obviously undesirable.
While all prior art actuators provided for a gravity "scram," that is the full insertion of all absorber elements solely under the influence of gravity in the case of a system malfunction, prior systems which have attempted to also incorporate a faster acting power "scram" have been characterized by undue complexity. Ideally a power "scram" should be achieved by operating the minimum number of components.